As disclosed, for example, in Non-Patent Document 1 and so on, a formed material having a tubular body and a flange portion formed on an end portion of the body is manufactured by performing a drawing process. Since the body is formed by stretching a blank metal sheet in the drawing process, the thickness of the circumferential wall of the body is usually less than that of the blank sheet. On the other hand, since the region of the metal sheet corresponding to the flange shrinks as a whole in response to the formation of the body, the flange thickness is larger than that of the blank sheet.
The abovementioned formed material can be used as the motor case disclosed, for example, in Patent Document 1 and so on. In this case, the circumferential wall of the body is expected to function as a shielding material that prevents magnetic leakage to the outside of the motor case. In some motor structures, the circumferential wall is also expected to function as a back yoke of a stator. The performance of the circumferential wall as the shield material or back yoke is improved as the thickness thereof increases. Therefore, when a formed material is manufactured by drawing, as described hereinabove, a blank metal sheet with a thickness larger than the necessary thickness of the circumferential wall is selected in consideration of the reduction in thickness caused by the drawing process. Meanwhile, the flange is most often used for mounting the motor case on the mounting object. Therefore, the flange is expected to have a certain strength.
With the abovementioned conventional formed material manufacturing method, a formed material having a tubular body and a flange formed at the end of the body is manufactured by drawing. Therefore, the flange thickness becomes larger than the blank sheet thickness. As a result, the thickness required for the flange to demonstrate the expected performance is sometimes exceeded and the flange becomes unnecessarily thick. Further, as a result of selecting a blank metal sheet with a thickness larger than the required thickness of the circumferential wall of the body, the thickness is unnecessarily increased up to that of the top wall of the body which makes little contribution to the motor performance. This means that the formed material is unnecessarily increased in weight and becomes unsuitable for applications that require lightweight motor cases. Further, with the conventional method, since a comparatively thick blank metal material is used, the material cost is increased.
Accordingly, Patent Document 2 and so on disclose a mold for performing compression drawing in a multistage drawing process as means for preventing the body of the drawn member from thinning.
In the compression drawing mold, a cylindrical member molded in a preceding step is fitted, in a state in which the opening flange portion thereof faces downward, onto a deformation-preventing member provided in a lower mold, the opening flange portion is positioned in a plate recess provided in the lower mold, and the outer periphery thereof is engaged with the recess. An upper mold is then lowered and the cylindrical portion of the cylindrical member is press fitted into a die hole provided in the upper mold, thereby inducing a compressive force and performing the compression drawing processing.
Since the deformation-preventing member in this case can be moved in the vertical direction with respect to the plate, the side wall of the cylindrical member receives practically no tensile force and can be prevented from thinning.
The compressive force applied in this case to a body preform is equal to the deformation resistance of the body preform at the time of press fitting into the die hole. Thus, the factors contributing to thickening are the mold clearance between the die and the punch, the die shoulder radius, and the material strength [(proof stress)×(cross-sectional area)] of the body preform which mainly relate to deformation resistance.    Non-Patent Document 1: “Basics of Plastic Forming”, Masao Murakawa and three others, First Edition, SANGYO-TOSHO Publishing Co. Ltd., Jan. 16, 1990, pp. 104 to 107    Patent Document 1: Japanese Patent Application Publication No. 2013-51765    Patent Document 2: Japanese Patent Application Publication No. H4-43415